Mitochondria exist in virtually all eukaryotic cells, and are essential to cell survival by producing adenosine triphosphate (ATP) via oxidative phosphorylation. Interruption of this vital function can lead to cell death.
Mitochondria also play a major role in intracellular calcium regulation by accumulating calcium (Ca2+). Accumulation of calcium occurs in the mitochondrial matrix through a membrane potential-driven uniporter.
The uptake of calcium activates mitochondrial dehydrogenases, and may be important in sustaining energy production and oxidative phosphorylation. In addition, the mitochondria serve as a sink for excessive cytosolic Ca2+, thus protecting the cell from Ca2+ overload and necrotic death.
Ischemia or hypoglycemia can lead to mitochondrial dysfunction, including ATP hydrolysis and Ca2+ overload. The dysfunction causes mitochondrial permeability transition (MPT). MPT is characterized by uncoupling of oxidative phosphorylation, loss of mitochondrial membrane potential, increased permeability of the inner membrane and swelling.
In addition, the mitochondria intermembrane space is a reservoir of apoptogenic proteins. Therefore, the loss of mitochondrial potential and MPT can lead to release of apoptogenic proteins into the cytoplasm. Not surprisingly, there is accumulating evidence that MPT is involved in necrotic and apoptotic cell death (Crompton, Biochem J. 341:233-249, 1999). Milder forms of cellular insult may lead to apoptosis rather than necrosis.
Cyclosporin can inhibit MPT. Blockade of MPT by cyclosporin A can inhibit apoptosis in several cell types, including cells undergoing ischemia, hypoxia, Ca2+ overload and oxidative stress (Kroemer et al., Annu Rev Physiol. 60:619-642, 1998).
Cyclosporin A, however, is less than optimal as a treatment drug against necrotic and apoptotic cell death. For example, cyclosporin A does not specifically target the mitochondria. In addition, it is poorly delivered to the brain. Furthermore, the utility of cyclosporin A is reduced by its immunosuppressant activity.
The tetrapeptide [Dmt1]DALDA (2′,6′-dimethyltyrosine-D-Arg-Phe-Lys-NH2; SS-02) has a molecular weight of 640 and carries a net three positive charge at physiological pH. [Dmt1]DALDA readily penetrates the plasma membrane of several mammalian cell types in an energy-independent manner (Zhao et al., J Pharmacol Exp Ther. 304:425-432, 2003) and penetrates the blood-brain barrier (Zhao et al., J Pharmacol Exp Ther. 302:188-196, 2002). Although [Dmt1]DALDA has been shown to be a potent mu-opioid receptor agonist, its utility has not been expanded to include the inhibition of MPT.
Thus, there is a need to inhibit MPT in conditions such as ischemia-reperfusion, hypoxia, hypoglycemia, and other diseases and conditions which result in pathological changes as a result of the permeability transitioning of the mitochondrial membranes. Such diseases and conditions include many of the common neurodegenerative diseases.